Exhaust System for Polaris 250cc Two Stroke

Hi Joe, have you talked to Polaris about your problem? They might have a performance spec. sheets that details the exhaust requirements. It's not uncommon for an engine of your size to require an expansion chamber.

you'll need to adjust the carb, most likely

or adapt the original pipe from the polaris

more information about the rotax & the Polaris, would help

My limited two stroke background tells me that Exhaust system design is HIGHLY CRITICAL ! Get good advice here, but do expect it to be more than trivial.

i agree. having a tuned exhast system is critical. exhast systems must be tuned for each model of engine, 2 cycle engines are very sensative as far as having the correct intake and exhaust tuning because different engines resonate at differnt frequecies. i hope i was'nt to trivial.

Joe

The rotex engine if I remember correctly has a little more HP. First check your spark plugs to see if your running lean or rich. You may have to make a carb adjustment. Second is the Polaris engine new? If not check compression. I belive the exhaust should be fine from the Rotex engine.

I hope this information is of use.

P.S. You should be running a Low Ash API-TC oil for this engine and NOT TCW 3 ashless oil. TCW 3 oils are for lower operating temperature applications (water cooled engines).

Good Luck

Dave

If the Polaris engine was designed to be used with an expansion chamber exhaust then it needs a similar one now or its output characteristics would be quite different.

The first thing I would try to do is get my hands on the chamber the Polaris was intended to be used with. If its original installation was a 4 wheeler, maybe ebay or craiglist would turn one up. If I couldn't buy one would it be possible to compare its functional dimensions to the one on your original rotax? This would to a large extent depend on if they were in a similar state of tune.

To even begin to see if an expansion chamber change is working you would likely have to tune the carburetor for the change. Does the engine in the current configuration wind out? Does it blubber at low rpm? Are there any signs of overheating such as pinging? Tuning or rejetting a carburetor on a two stroke is not hard for a technically minded person. Many articles have been written about it in motorcycle publications. Carb tuning after a pipe change is especially important on a two stroke since the pipe (expansion chamber) forces some of the unburned exhaust gasses back into the cylinder before the exhaust port closes.

As for designing then building an expansion chamber this article is very good. http://vincentcrabtree.co.uk/2TExpChambers.aspx

The original manufactures use similar methods but then back it up with a dyno.

After you have played with the equations you may choose to stick with the rotax pipe but just try to move the tuned pipe rpm to more closely match the Polaris engine. This may be possible by just changing the length of the header section.

To see if your perceived power loss is accurate, I would do a search for dyno charts of both engines on the internet. This would tell you if this is even correctable with an exhaust change.

Exhaust tuning is critical in a two stroke, but not difficult to understand why. The way an expansion chamber works is the exhaust pops out of the exhaust port and then expands into the expansion chamber volume. But ahead of that expansion is the sound pressure wave of the initial opening pop. That wave has already entered and bounced off the rear of the chamber. At that end there is a narrow tube called the stinger, and that tube can be either moved or lengthened or shortened to change the timing of the return pressure wave. That timing is related to the RPMs of the motor and should be set so that the highest pressure point of the wave is bouncing back to the opening on the cylinder just as the piston is closing the port off for the next power stroke.

The sound wave and the pressure wave should meet at the piston/port at the same time. This action forces gas/air mixture from just outside the port, back into the cylinder and raises the internal pressure of the next compression stroke. At this moment, the engine is said to be "on the pipe".

In essence, you are blowing a flute with a motor and tuning it for highest volume AT THAT FREQUENCY.

So, if your engine must run within a narrow range of speeds, then the tuning of the exhaust is critical, but for two engines of the same displacement, the only adjustment you will likely need to do is to move the stinger or change its length so that the timing of the wave is correct. Like moving the slider on a trombone! Longer means lower frequency-slower engine speed on-the-pipe. Shorter means higher frequency-higher engine speed on-the-pipe.

I learned to port, polish, and tune two strokes from a master back in the day....

That's exactly the point I was trying get across in my post #1. Just because the engines may be the same displacement doesn't mean their exhaust systems are compatible, without doing some adjustments.

Another adjustment besides moving the stinger or changing its length is to shave the head to increase compression, and, if it is not a reed valve intake, to shave the skirt of the piston a mm or two to increase the intake duration timing. You can't undo this, so be sure you want a longer intake period. You have to look at the opening and closing of all the ports and make sure the intake open and close do not overlap with the charge port opening and closing. If you overlap them it has the opposite effect. But if the intake port closes just prior to the charge ports opening, then the crankcase pressure will still be low when at that moment you want it to be increasing. But if there is a few degrees between these points, then you may have some room to play.

The piston skirt should close the intake in time to build a little pressure in the crankcase prior to the charge port opening so that there is a positive pressure to scavenge the last of the exhaust and fill the combustion chamber with a new charge. Conversely, the skirt should begin opening the intake just after the charge ports close, so that a lower pressure can form in the crankcase and increase the amount of charge is drawn from the carb. It's tricky business cutting the skirt, so unless you've done it before, don't do it.

Thank You Cr4 Contributors,

The information that has been supplied has been very interesting and informative.

I don't know if I can work out DaveB advise, but I will work on this with a couple of my friends.

Thanks for all your submissions they have been much appreciated.

Best Regards

Joe